1. Field of the Invention
The present invention relates to a developing device used in an electrophotographic field, wherein an electrostatic latent image is visually developed by using a non-magnetic one-component developer.
2. Description of the Related Art
As is well known, an electrophotographic printer carries out the processes of: producing a uniform distribution of electrical charges on a surface of an electrostatic latent image formation body; forming an electrostatic latent image on the electrically charged surface of the electrostatic latent image formation body by optically writing an image thereon by using a laser beam scanner, an LED (light emitting diode) array, a liquid crystal shutter array or the like; visually developing the electrostatic latent image with a developer, i.e., toner, which is electrically charged to be electrostatically adhered to the electrostatic latent image zone; electrostatically transferring the developed visible image to a paper; and fixing the transferred image on the paper. Typically, the electrostatic latent image formation body may be an electrophotographic photoreceptor, usually formed as a drum, called a photosensitive drum, having a cylindrical conductive substrate and a photoconductive insulating film bonded to a cylindrical surface thereof.
In the developing process, a two-component developer composed of a toner component (colored fine synthetic resin particles) and a magnetic component (magnetic fine carriers) is widely used, as it enables a stable development of the latent image. Note, typically the toner particles have an average diameter of about 10 .mu.m, and the magnetic carriers have a diameter ten times larger than the average diameter of the toner particles. Usually, a developing device using the two-component developer includes a vessel for holding the two-component developer, wherein the developer is agitated by an agitator provided therein. This agitation causes the toner particles and the magnetic carriers to be subjected to triboelectrification, whereby the toner particles are electrostatically adhered to each of the magnetic carriers. The developing device also includes a magnetic roller provided within the vessel as a developing roller in such a manner that a portion of the magnetic roller is exposed therefrom and faces the surface of the photosensitive drum. The magnetic carriers with the toner particles are magnetically adhered to the surface of the magnetic roller to form a magnetic brush therearound, and by rotating the magnetic roller carrying the magnetic brush, the toner particles are brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon. In this developing device, a ratio between the toner and magnetic components of the developer body held in the vessel must fall within a predetermined range, to continuously maintain a stable development process. Accordingly, the developing device is provided with a toner supplier from which a toner component is supplied to the two-component developer held in the vessel, to supplement the toner component as it is consumed during the development process, whereby the component ratio of the two-component developer held by the vessel is kept within the predetermined range. This use of a two-component developer is advantageous in that a stable development process is obtained thereby, but the developing device per se has the disadvantages of a cumbersome control of a suitable component ratio of the two-component developer, and an inability to reduce the size of the developing device due to the need to incorporate the toner supplier therein.
A one-component developer is also known in this field, and a developing device using same does not suffer from the above-mentioned disadvantages of the developing device using the two-component developer, because the one-component developer is composed of only a toner component (colored fine synthetic resin particles). Two types of the one-component developer are known; a magnetic type and a non-magnetic type. A developing device using the magnetic type one-component developer can be constructed in substantially the same manner as that using the two-component developer. Namely, the magnetic type one-component developer also can be brought to the surface of the photosensitive drum by a rotating magnetic roller as in the developing device using the two-component developer. The magnetic type one-component developer is suitable for achromatic color (black) printing, but is not suitable for chromatic color printing. This is because each of the toner particles of which the magnetic type one-component developer is composed includes fine magnetic powders having a dark color. In particular, the chromatic color printing obtained from the magnetic type one-component developer appears dark and dull, due to the fine magnetic powders included therein. Conversely, the non-magnetic type one-component developer is particularly suitable for chromatic color printing because it does not include a substance having a dark color, but the non-magnetic type one-component developer cannot be brought to the surface of the photosensitive drum by the magnetic roller as mentioned above.
A developing device using the non-magnetic type one-component developer is also known, as disclosed in U.S. Pat. No. 3,152,012 and U.S. Pat. No. 3,754,963. This developing device includes a vessel for holding the non-magnetic type one-component developer, and a conductive elastic solid roller provided within the vessel as a developing roller in such a manner that a portion of the elastic roller is exposed therefrom and can be pressed against the surface of the photosensitive drum. The conductive elastic solid developing roller may be formed of a conductive silicone rubber material or a conductive polyurethane rubber material, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 60-12627 and Japanese Unexamined Patent Publications (Kokai) No. 62-118372 and No. 63-189876. When the conductive solid rubber roller is rotated within the body of the non-magnetic type one-component developer held by the vessel, the toner particles composing the non-magnetic type one-component developer are frictionally entrained by the surface of the conductive solid rubber developing roller to form a developer layer therearound, whereby the toner particles can be brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon. The developing device further includes a blade member engaged with the surface of the developing roller, to uniformly regulate a thickness of the developer layer formed therearound so that an even development of the latent image can be carried out. The blade member also serves to electrically charge the toner particles by a triboelectrification therebetween. In this developing device, the development process is carried out in such a manner that, at the area of contact between the photosensitive drum and the conductive solid rubber developing roller carrying the developer layer, the charged toner particles are electrostatically attracted and adhered to the latent image due to a developing or bias voltage applied to the conductive solid rubber developing roller.
In the developing process, the remaining toner particles not used for the development of the latent image should be once removed from the developing roller, and then a fresh developer layer be formed thereon. This is because the developer layer formed of the remaining toner particles is subjected to physical and electrical affects during the developing process, to thereby hinder a physically and electrostatically even formation of the developer layer around the developing roller. As is obvious, the physically and electrostatically uneven formation of the developer layer around the developing roller will cause an uneven development of a latent image. Japanese Unexamined Patent Publication No. 61(1986)-43767 discloses the utilization of a toner-removing roller for removing the remaining toner particles from the developing roller. This toner-removing roller is made of a suitable conductive sponge rubber material, and is disposed to be in contact with the developing roller. The toner-removing roller is rotated in the same direction as the developing roller so that the surfaces of the toner-removing roller and the developing roller are rubbed against each other in counter directions at the contact area therebetween, whereby the remaining toner particles are mechanically removed from the developing roller. Also, a bias voltage is applied to the toner-removing roller so that the remaining toner particles are electrostatically attracted from the developing roller to the toner-removing roller.
Note, the prior toner-removing roller as mentioned above has a disadvantage of an early hardening thereof due to a penetration of the toner particles into the sponge structure of the toner-removing roller. When a hardness of the toner-removing roller becomes larger, i.e., when the toner-removing roller loses its softness, the remaining toner particles are squashed on the surface of the developing roller rather than being removed therefrom, and thus the squashed toner particles are firmly adhered to the surface of the developing roller, so that a proper development of a latent image cannot be obtained.